1. Field of the Invention
This invention relates to a sealed storage battery with a safety vent and also to a method of producing such a battery.
2. Description of the Related Art
Recently, storage batteries or cells have been used in various appliances including an electric home appliance and an electric car, and above all there has been a strong demand for medium-sized and large-sized cells (generally, the capacity of a medium-sized cell is 10 to 100 Ah, and the capacity of a large-sized cell is 100 Ah or more) with a high energy density and high reliability, used as a movement-purpose power source for driving or moving an electric car or the like. A nickel-cadmium storage battery of the vent type and a lead storage battery of the vent type have heretofore been used as such a medium- or large-sized cell for energy storage purposes and UPS purposes. However, it is important that the movement-purpose power source should have a high energy density.
To meet this requirement, and more specifically to improve the energy density and the service life, attention has now been directed to a nickel-hydrogen storage battery instead of a nickel-cadmium storage battery and a lead storage battery. One characteristic of this nickel-hydrogen storage battery is that gas is produced in a battery case when the battery is charged and discharged, and therefore the battery is sealed so as to prevent the gas from leaking to the exterior of the battery, thereby preventing an electrolyte from being dried up.
In order to form a medium- or large-sized cell with a high energy density and a long service life, which is represented by a nickel-hydrogen storage battery, into a sealed construction, it is necessary to confine gas, produced during charging and discharging, within the cell, and it is also necessary to accurately set an operating pressure of a safety vent, which serves to discharge this gas in the event of an abnormal condition, to 2 to 8 kg/cm.sup.2. The operating pressure of the simple-type, e.g. cap-type, safety vent has varied greatly, so that the performance of the battery could not be fully achieved. And besides, since such a high pressure develops within the battery, a strength of fixing of a fixing portion between the safety vent and a battery case lid, as well as a hermetic seal of this fixing portion, must be improved.
Furthermore, if the operating pressure of the safety vent is less than 2 kg/cm.sup.2, the valve operates even in a normal condition, so that the battery can not be kept in a sealed condition, and in contrast if the operating pressure is more than 8 kg/cm.sup.2, there is a possibility that the battery case is ruptured. Therefore, it is important that the operating pressure of the safety vent should be accurately set to the predetermined range, and it has been essential to check the operating pressure of the safety vent before the safety valve is mounted on the battery case.
However, in the type of construction in which the safety valve of the assembling type is fixedly secured to the battery case by screw tightening, there is a possibility that screw-fastened portions become loose by vibrations of the moving body (e.g. vehicle) and the expansion and contraction of the lid due to a change of the internal pressure during the charging and discharging of the battery, so that there is a possibility that the gas and the electrolyte leak to the exterior. When the safety vent is mounted on the battery case through a packing of rubber or the like so as to enhance the hermetic seal, the packing, firmly clamped for a long time period, is affected by aging and thermal deterioration, which results in a possibility that the hermetic effect is lost, so that the gas, produced within the battery, as well as the electrolyte, leaks to the exterior.
In the type of construction in which the safety valve of the assembling type is fixedly fitted in the battery case, a fixing strength and a hermetic effect are lower than in the above construction in which the safety vent is fixed to the battery case by screw tightening, and therefore there is a possibility that the gas, produced within the battery, as well as the electrolyte, leaks to the exterior.